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Understanding the Stars. How can we learn about the lives of stars, which last millions to billions of years? Consider the story of the Ephemera Image from: http://www.startunzflutes.com/graphics. How can we Study the Life Cycles of Stars?. A star can live for millions to billions of years.

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understanding the stars
Understanding the Stars
  • How can we learn about the lives of stars, which last millions to billions of years?
  • Consider the story of the Ephemera
  • Image from: http://www.startunzflutes.com/graphics
how can we study the life cycles of stars
How can we Study the Life Cycles of Stars?
  • A star can live for millions to billions of years.
    • we will never observe a particular star evolve from birth to death
    • The key is that all stars were not born at the same time.
    • the stars which we see today are at different stages in their lives
    • we observe only a brief moment in any one star’s life
    • by studying large numbers of stars, we get a “snapshot” of one moment in the history of the stellar community
    • we can draw conclusions just like we would with human census data…we do stellar demographics!
classification of stars
Classification of Stars
  • Stars were originally classified based on:
    • their brightness
    • their location in the sky
  • This classification is still reflected in the names of the brightest stars…those we can see with our eyes:

Order of brightness within a constellation

Latin Genitive of the constellation

 Orionis

 Geminorum

classification of stars1
Classification of Stars
  • The old classification scheme told us little about a star’s true (physical) nature.
    • a star could be very bright because is was very close to us; not because it was truly bright
    • two stars in the same constellation might not be close to each other; one could be much farther away
  • In the 20th Century, astronomers developed a more appropriate classification system based on:
    • a star’s luminosity
    • a star’s surfacetemperature
    • A star’s stage of life
summer triangle
Summer Triangle
  • Which of these three Stars in the Summer Triangle is the brightest (to the naked eye)
  • Which is the most Luminous?
  • Which of these three are the closest?
don t always trust your eyes
Don’t always trust your eyes!
  • Note that Deneb is a bit larger than the other two
  • Its also a bit farther away—2250 light years!
  • Vega is 22 light years away—its in the neighborhood
  • Note that Altair (about 30 lyrs) is probably flattened..its rotating very fast!
  • Apparent brightness depends on size, temp, and distance!
luminosity and intensity
Luminosity and Intensity

Our goals for learning:

  • What is luminosity and how do we determine it?
  • How do we measure the distance to nearby stars?
  • How does the magnitude of a star depend on its apparent brightness?
luminosity of stars
Luminosity of Stars

Luminosity – the total amount of power radiated by a star into space.

  • Apparent brightness refers to the amount of a star’s light which reaches us per unit area.
    • the farther away a star is, the fainter it appears to us
    • how much fainter it gets obeys an inverse square law
    • its apparent brightness decreases as the (distance)2
    • Apprent brightness is also known as the Intensity
  • The apparent brightness of a star depends on two things:
    • How much light is it emitting: luminosity (L) [watts]
    • How far away is it: distance (d) [meters]

App Bright = L / 4d2

apparent brightness
Apparent Brightness

The Inverse Square Law for Light

What Determines Apparent Brightness?

measuring distances to stars
Measuring Distances to Stars

parallax – apparent wobble of a star due to the Earth’s orbiting of the Sun

measuring distances to stars1
Measuring Distances to Stars

p = parallax angle

d = 1 AU/ p

Gives distance in


convert p into arcsec

d = 206,265 AU/ p

measuring distances to stars2
Measuring Distances to Stars

let’s define

1 parsec  206,265 A.U. = 3.26 light years

d = 1 / p

If p is in arcsec and d is in parsecs

A star with a parallax of 1 arcsec is 1 parsec distant

the brightness of stars
The Brightness of Stars

Astronomers still use an ancient method for measuring stellar brightness which was proposed by the Greek astronomer Hipparchus (c. 190 – 120 B.C.)

Magnitude Scale

This scale runs backwards:

The bigger the number, the fainter the star

Brightest stars are #1, next brightest are #2, etc.

the modern magnitude system
The Modern Magnitude System

apparent magnitude

= -2.5 log (app bright)

  • brightness of a star as it appears from Earth
  • each step in magnitude is 2.5 times in brightness

absolute magnitude

  • the apparent magnitude a star would have if it were 10 pc away
what good is this
What good is this?
  • If you know apparent brightness, you can find magnitude.
  • If you know magnitude, you can use another relationship to find distance
  • M – m = 5 – 5log(d)
  • M= Absolute magitude
  • M = m when distance is ten parsecs.
an example of how this works
An example of how this works!
  • Deneb has an apparent visual magnitude of

1.26 (see chart of brightest stars at end of your text)

Deneb has an Absolute visual magnitude of -8.73

(this is about the same brightness as the quarter moon---but at 32.6 light years away!)

Using the weird equation, the distance to deneb can be calculated: 2500 light years (M – m = 5 – 5log(d))

One last obvious question: How did we ever know the Absolute visual magnitude to Deneb without knowing its distance in the first place?

16 3 classifying stars
16.3 Classifying Stars
  • Hypothysis:
  • the Luminoisty and Abolsute Magnitude of Stars can be known if we know their Classification.
  • There Classification is completely revealed in their Spectra.
  • Comparing the spectra of nearby stars allows us to test this hypothis.
  • The Classification is known as Spectral Type
spectral type is revealed in the colors of stars
Spectral type is revealed in the Colors of Stars

Stars come in many different colors.

The color tells us the star’s temperature according to Wien’s Law.

Bluer means hotter!

spectral type classification system
Spectral Type Classification System



Oh Be A Fine Girl/Guy, Kiss Me!

50,000 K 3,000 K


spectral types of stars2
Spectral Types of Stars
  • Spectral types are defined by the:
    • existence of absorption lines belonging to various elements, ions, & molecules in a star’s spectrum
    • the relative strengths of these line
  • However, spectral type is not determined by a star’s composition.
    • all stars are made primarily of Hydrogen & Helium
  • Spectral type is determined by a star’s surface temperature.
    • temperature dictates the energy states of electrons in atoms
    • temperature dictates the types of ions or molecules which exist
    • this, in turn, determines the number and relative strengths of absorption lines in the star’s spectrum
    • this fact was discovered by Cecilia Payne-Gaposchkin in 1925
spectroscopic parllax
Spectroscopic parllax
  • Summary:
  • If we know distance (d) from parallax measurements and..
  • If we know apparent visual magnitude (m) from photometry or image size (apparent brightness is also measured in this way).
  • Then we can calculate Absolute visual magniutde (M). Luminosity is also measured in this way.
  • We can obtain spectra and spectral type for all these nearby stars (about 10,000!)
  • We can make a table that provides Absolute visual magnitude for stars given their spectral type.
  • With this table, we can find the distance to distant stars simply by obtaining their spectra and apparent visual magnitude.
  • In a strange way, we have extended the parallax measurements out way beyond the one hundred parsec limit!